The invention relates to equipment for the retransmission of television signals in a common channel.
The ratio between the peak amplitude of the video component and the peak amplitude of the audio component modulated in frequency in a composite television signal is defined in place of the transmission. In retransmission equipment, for example in rebroadcasting transmitters, transposers or translators, it is conventional practice to simultaneously amplify in a so-called "common" amplifier chain, the video and audio components of the television signal, automatic gain control loops of the common amplifiers acting simultaneously on the two components.
A television signal transmitted under good conditions, with correct nominal levels for the video and audio components, can undergo selective frequency attenuations as a result of propagation phenomena. These selective attenuations modify the video/audio ratio in the composite signal received by the retransmission equipment. Therefore such equipment is linear and the rebroadcast signals are the image of the signals received.
Retransmission equipment in which the gain of the common amplifiers is controlled from the peak amplitude of the video component will not affect the video/audio ratio which, if it is abnormal at the input, will be retransmitted as it is.
In the same way, retransmission equipment in which the gain of the common amplifiers is controlled on the basis of the sum of the peak amplitudes of the video and audio components, will control a gain modification affecting the two components in the same way, even when the video/audio ratio is abnormal due to the attenuation of one of the components, or will maintain the gain unchanged if the sum of the video and audio peak values is constant, although the video/audio ratio is abnormal.
The only way in which this disadvantage can be obviated is to completely demodulate the signal, restore the levels of the video and audio components, to their nominal values and then rebroadcast them under the same conditions as governed them during their initial transmission. This procedure is complicated and costly.